Structuring material by a direct process of ablation by pulsed laser beams is a well established technology used widely for the production of precision devices in, but not limited to, the medical, automotive, solar, display and semi conductor industries.
The ablation process involves the exposure of a material surface to one or more pulses of intense radiation generated by a pulsed laser source. If the laser wavelength is such that the radiation is strongly absorbed in the top layer of the material and the energy density high enough so that the absorbed energy raises the temperature of the top layer to well above the melting point of the material then in this case this top layer of material is decomposed and changed into gaseous, liquid or solid particle by-products that expand from the surface. The essential requirement for the process of ablation to occur is that enough energy is absorbed in the material in a sufficiently short time that the temperature is raised rapidly to a point such that the material decomposes.
For ablating thick materials each laser pulse removes between 50 nm and several microns of material depending on the energy density, laser wavelength and material absorption co-efficient. Each pulse behaves in the same way so that after a succession of pulses fractions of a millimeter of material can be removed. The ablated material is often converted to a gaseous material but in many cases can include both liquid and solid constituents.
For thin films of material the ablation process can be somewhat different. Where the film is deposited on top of a substrate made of a different material and the thickness of the film is low (e.g. less than 1 micron) it is possible for ablation to be undertaken by one of two methods. If the film absorbs the laser radiation strongly then no radiation penetrates to the lower substrate but is absorbed within the film. Such strong absorption in a thin layer causes the temperature of the film to rise rapidly and heat is conducted to the lower side where it causes the disruption of the bond between the film and the lower substrate. Such a process occurs with thin metal films. In this case the metal is removed in one laser pulse in the form of a mixture of particles and liquid.
In the case where the film is wholly or partially transparent to the laser radiation and the lower substrate absorbs the radiation more strongly than the film, the energy is absorbed at the top of the lower substrate at the interface between the two layers causing a rapid temperature rise and the ablation of the top layer. In this case the removed top layer is generally decomposed into particles ranging in size from sub micron to many tens of microns.
If the underlying substrate material is transparent to the laser radiation and the thin film absorbs it then it is sometimes advantageous to bring the laser beam to the substrate/film interface directly through the substrate. In such cases the film is often delaminated from the substrate in only one laser shot of modest energy density.
All processes of material ablation by laser lead to the generation of a range of ablation product components which can be in gaseous, liquid or solid form. These include atoms, molecules, clusters, particles, polymer chains, small and large material fragments, liquid droplets and jets and others. We refer hereafter to this material as ablation debris. The control of this ablation debris is a significant problem and the deposition of ablation debris onto the substrate surface has to be minimised to avoid contamination. In particular in the case of thin film ablation for FPD manufacture where the direct laser ablation process replaces a wet chemical or plasma etching process (in which particulate contamination does not readily occur) the re-deposition of ablation debris onto the surface of the substrate during a laser ablation FPD production process cannot be tolerated. It is an object of the present invention to control the flow of the ablation debris from the substrate surface and minimize it's re-deposition onto the substrate.
Methods have been used before to attempt to capture and control ablation debris generated during laser ablation processes. Most of these rely on some type of gas flow near the surface that is being ablated. The flow is often directed along the surface and can be created by blowing on one side of the area and sucking strongly from the other. The gas used is often air but in some cases other gases such as helium, oxygen or argon are used. In all cases the flow of gas is used to redirect the moving ablation debris and either direct it away from the critical area or preferably remove it totally from the substrate area. The process relies on momentum exchange between the gas molecules and the ablation debris and hence high pressures and high gas flow rates are needed for it to be effective. The use of a heavy gas such as argon can aid this process. If helium is used the effect is different as the mass of helium molecules is much less than of air molecules, and so helium is less effective than air in interacting with the ablation debris. In this case the moving ablation debris can then travel further from the ablation site before being slowed and deposited. This has the effect of moving the deposited material further from the site of origin but does not significantly reduce the total amount of material re-deposited.
The use of a reactive gas such as oxygen can reduce the amount of deposited material where the ablation debris reacts with the reactive gas to transform it to a pure gas. An example of this is the ablation of some polymer materials. Here the organic particles created can react with the oxygen to form pure gases such as carbon dioxide or carbon monoxide.
A liquid flow across a surface is sometimes used as an alternative to a gas flow to entrap ablation debris. During the laser ablation process a thin layer of water, or other liquid, is directed across the surface of the ablation region. The layer is required to be thin so that it does not absorb or disturb the incoming laser beam and is generally created by some type of atomizer nozzle located on one side of the ablation region. Such a system has been described recently in Clean Laser Machining (Industrial Laser Solutions, May 2003). Having passed across the substrate surface the fluid is collected in some type of channel around a chuck holding the substrate.
The methods listed above make use of unconstrained gas or liquid flows directed across a surface. Such usage is of limited effectiveness in removing the ablation debris since the capture of the debris is not totally effective and re-deposition in other areas of the substrate often occurs. Ablated debris is simply blown or flowed to another area of the substrate where it re-deposits. Another serious disadvantage of the liquid flow method is that it is inappropriate for dealing with large substrates associated with FPD manufacture since in this case a mounting chuck for the substrate can be very large and any water capture channels are a long way from the ablation point, As a result the re-deposition of ablation debris from the fluid flow onto the substrate is likely.
It is an object of the present invention to avoid these limitations and provide for removal of ablation debris from the surface of substrates of any size without significant re-deposition on the surface.